Error: 'Contact' entity doesn't contain attribute with Name = ''

Hi, I'm retrieving all contacts that don't have birthdate equals to birthday. But it's giving me an error: " 'Contact' entity doesn't contain attribute with Name = 'new_BirthDay'.

when in fact it does contain an attribute "new_birthday".

Can anyone help me to solve the error ?

private void button1_Click(object sender, EventArgs e)
{

_service = GetService();

var Criteria = new FilterExpression();

contact = new Entity("contact");
QueryExpression query = new QueryExpression("contact");
query.ColumnSet.AddColumns("new_BirthDay", "birthdate");
FilterExpression childFilter = query.Criteria.AddFilter(LogicalOperator.Or);
ColumnSet cols = new ColumnSet(new[] { "new_BirthDay", "birthdate" });

childFilter.AddCondition("new_BirthDay",ConditionOperator.NotEqual, "birthdate");

//int value = ((OptionSetValue)contact["new_BirthDay"]).Value;

// string optionsetText = contact.FormattedValues["new_BirthDay"];

// Pass query to service

EntityCollection results = _service.RetrieveMultiple(query);

richTextBox1.Text = "";
foreach (var res in results.Entities)
{
richTextBox1.Text += res.GetAttributeValue<string>("new_BirthDay");
}